1. Field of the Invention
This invention relates to a method for manufacturing a cushion material of variable hardness used in car seats or beds.
2. Description of the Related Art
When a cushion material is used for car seat padding, a material M having for example the structure shown in FIG. 15 is desirable in order to improve the comfort of the passengers. This cushion material m is so formed that the surface of the material on which the load acts, i.e. the surface on which the passenger sits, has a low hardness A, and the hardness of the material progressively increases to a medium hardness B and greater hardness C towards the lower part of the material (this variation of hardness is defined as a "hardness gradient").
Conventionally, the following methods were used for manufacturing a cushion material M having a predetermined hardness gradient.
In the first of these methods, layers of cushion material of different hardness were stuck together-using an adhesive, e.g. a liquid urethane adhesive. This method however involves a complex process, and manufacturing costs could not be reduced.
In the second method, a mold was filled with a mixed cotton containing a thermofusible fiber (i.e. a fiber having a core/sheath composite structure wherein the reel ting point of the sheath is no less than 30.degree. C. below that of the core melting fiber), and the cotton was subjected to thermal compression so as to form a cushion material (Japanese Patent Application Laid-Open No. 3-170112). This method overcomes the drawback of the first method.
In this second method, in order to obtain a cushion material having a different hardness in different parts of the material, the density is increased in certain regions by compressing the fiber base to different extents in different regions. Alternatively, a cushion material having a lower hardness in some regions than others was obtained by lining predetermined regions of a mold with a thermofusible fiber that had been given a suitable degree of support by a binder.
By varying the compression applied to the thermofusible fiber to be molded by heating, the density of the cushion material can be varied in different parts of the material, and a cushion material of varying hardness can be obtained. The aforesaid Patent Publication does not however disclose how to make the density of the cushion material vary continuously, and the thermal compression is also difficult to manufacture the cushion material having a continuously varying density.
Although it is generally possible to adjust the fiber content using a thermofusible fiber supported by a binder, it renders the manufacturing process complex. Further, in high hardness parts of a cushion material obtained by this method, the thermofusible fiber content is high and so the high thermofusible fiber content leads to a decrease of elasticity.